The use of plastics in various industries is steadily increasing due to their light weight and continual improvements to their properties. For example, in the automotive industry, polymer-based materials may comprise a significant portion, e.g., at least 15 percent, of a given vehicle's weight. These materials are used in various automotive components, such as, interior and exterior trim and side panels. As the industry seeks to improve fuel economy, more steel and aluminum parts may be targeted for replacement by polymer-based materials. In addition, improvements in the mechanical properties of polymers are necessary in order to meet more stringent performance requirements. Such mechanical properties include, for example, stiffness, dimensional stability, modulus, heat deflection temperature, barrier properties, and rust and dent resistance. Improved mechanical properties may reduce manufacturing costs by reducing the part thickness and weight of the manufactured part and the manufacturing time thereof.
There are a number of ways to improve the properties of a polymer, including reinforcement with particulate fillers or glass fibers. Currently, it is known that polymers reinforced with nanometer-sized platelets or particles of layered silicates or clay can significantly improve the mechanical properties at much lower loading than conventional fillers. (See, for example, U.S. Pat. No. 6,469,073 issued to Manke et al. (2002) and U.S. Patent Application Publication No. US 2002/0082331 A1 to Mielewski et al. (2002).) This type of composite is termed a “nanocomposite.” More specifically, polymer-silicate nanocomposites are compositions in which nano-sized particles of a layered silicate, e.g., montmorillonite clay, are dispersed into a thermoplastic or a thermoset matrix. The improvement in mechanical properties of nanocomposites is believed to be due to factors such as the increased surface area of the particles.
However, in the development of polymer-silicate nanocomposites, structures yielding the best fire retardancy frequently produce unacceptable mechanical properties. Another problem with the polymer-silicate nanocomposites is the thermal instability of the organic ions introduced between the layers. That is, although polymer-silicate nanocomposites have been shown to exhibit a significant increase in thermal stability, they sometimes thermally decompose within a host polymer. (See, for example, J. Zhu, F. M. Uhl, A. B. Morgan, C. A. Wilkie, Chem. Mater., 13, 4649–4654 (2001).)
Thus, in recent years increasing attention has been devoted to developing polymer-graphite composites for applications where electrical conductivity or thermal conductivity enhancement is desired. Graphite, which like clay also has a layered structure, has proven to be a candidate for replacement of clays. Graphite, a refractory material, can provide excellent fire retardancy for polymers. Graphite nanocomposites also include the char formation, which is an additional advantage for fire retardancy. In addition to high electrical conductivities, polymer-graphite composites often possess other desirable properties, such as corrosion resistance, low cost and ease of processing. Given this combination of properties, polymer-graphite composites present an attractive alternative to metal conductors in certain applications.
However, challenges to developing such composites exist. That is, although several kinds of graphite intercalation compounds have been synthesized, only a few graphite-polymer nanocomposites have been reported because organic molecules are hard to directly intercalate into graphite. Moreover, current methods have proven to be timely and costly.
Current processes to prepare exfoliated or intercalated graphite involve relatively undesirable solvents and extreme conditions. For example, in one process, an artificial graphite is prepared by introducing sulfuric acid into the graphite interlayers and rapidly heating the graphite at temperatures of between 800° C. and 1,000° C. (See, for example, U.S. Pat. No. 5,482,798, issued to Mototani et al. (1996).) Graphite can be intercalated by exposure to an appropriate chemical reagent, known as the intercalate, which enters between the carbon layers of the graphite. The resulting material known as intercalated graphite layers comprising carbon are stacked on top of one another in a periodic fashion. Heating intercalated graphite layers to a sufficiently high temperature causes exfoliation, which is a sudden increase in the dimension perpendicular to the carbon layers of the intercalated graphite, forming vermicular or wormlike shapes. (See, for example, U.S. Pat. No. 4,946,892 issued to Chung (1990).) Thus, there remains a need for new methods for forming polymer graphite composites.